Retreading of tires



United States Patent O 3,330,891 RETREADING OF TIRES Jan HerbertFarqharson Kent, St. Martin, Jersey, Channel Islands, assignor toKentredder Limited, St. Helier,

Jersey, Channel Islands, a co'poration of Jersey, Channel Islands FiledFeb. 24, 1964, Ser. No. &16,739 1 Clai. (Cl. 264-336) This inventionrelates to an improved method of retreading tires.

In the specification of my prior British Patent No. 732,241, there isdescribed a method of retreading tires in which, instead of applying thenew tread band to the tire, an unvulcanised strip of rubber constitutingthe new tread is first placed in the matrix, following which the outercover of the tire is superimposed on the strip, the cover beingcontraoted by pressure applied to the beads to force them apart untilthe contraction is sufiicient to enable the cover to be introduced intothe matrix.

There is an increasing demand for tires of the type having a rigidbreaker, that is, in which the reinforcement of the tire carcass isbuilt up wholly or partially of steel wires or nylon thread members,which are rel atively inextensible compared with the textile threadsused in conventional tires. This results in a tire, which is highly-resistant to extension or compression along the line of itscircumference.

It follows therefore that, because an outstanding characteristic of therigid breaker type of tire is that it has virtually no circumfe'ent-ialStretch whatsoever, when the tire has been buffed and subsequently hasto be built up with new rubber in a conventional manner, as described inBritish specification No. 732,241, it cannot be inserted into a circularmould of the correct diameter without causing a crinkle to form in it.When the tire is infiated, or pressed out by other means against themould, the act of straightening out the crinkle disrupts the newunvulcanised tread rubber and thereby upsets its distribution.

This difficulty can be met by building the new tread onto the tire, butthis is not satisfactory mainly for the reason that the equipment iscomplicated, involving the use of a mould, which is segmental to enableit to be contracted onto the tire.

According to the present nvention a method of Ietreading a tire of therigid breaker type comprises the steps of:

(i) Placirg an unvulcanised tread band strip in the matrix (ii)Apply-ing pressure to the strip uniformly around its internal perimeterso that it is engaged with the tread pattern or contour of the matrixand (iii) Then inserting the outer cover, which has been prepared to apredetermined size and contour, in the usual manner, in the matrix andsuperimposing it on the tread band strip.

In order to apply the necessary pressure uniformly throughout thesurface of the rubber strip as it sinks into the pattern of the matrix(and consequently increase its circumference as it does so) someextensible means must be employed and a degree of rigidity is alsorequired if signs of the tread pattern of the mould are not be retainedon the inner surface of the compound after the pressure operation. Theelimination of any impresson of the tread pattern on this surface isimportant, for, if it is retained, there will be a tendency to trap airbetween the new rubberand the buffed surface of the tire to be retreadedand this tends to spoil the bond.

The invention is illustrated in the accompanying drawings in which:

FIGURE 1 is a sectional View showing the mould in ice which has beeninserted a strip of rubber to form the new tread band together with a'snker device including a contractile tire cover, an infiatable bag andring for positioning the new tread strip in the mould.

FIGURE 2 is a sectional view corresponding to FIG- URE 1 but showing thetread strip after it has been forced into position in the base wall ofthe mould and prior to removal of the snker device.

Referring to the drawings, in carrying out the retreading operation ofthis invention, a strip 1 of unvulcanised rubber constituting the newtread band is positioned within a two-part circular mould or matrix 2,the two parts of the mould having complementary circumferential fianges3, by which they are detachably Secured to one another by studs or bolts(not shown) to permit of removal of the tire following the retreadingoperation which tire, as explained above, is relatively inextensible byreason of the nature of the reinforcernent. As shown in the drawings,the mould matrix is symmetrical with respect to a central planecontaining the abutting faces -of the fianges 3. The strip 1 of treadband material is substantially wider transversely to that plane than itis thick radially of the matrix. When the strip 1 is in serted into thematrix, the strip extends substantially equally to both sides of theaforementioned plane.

An expandable member 10 termed the snker and consisting of aconventional smooth surfaced tire casing or carcass of the same size asthat for which the mould is designed, is then used as the Sinking meansto force the tread band 1 into position where it lies against theperimeter of the mould. The snker is introduced in the usual rnannerwhich consists in spreading the walls 'ap-art until there is sufiicientcontraction at the tread to enable it to be inserted into the mould,following which it is allowed to expand and thus perform its operationas a snker to apply pressure to the tread band and force it intoengagement with the mould.

A worn textile tire can be used for the snker, the tire, i.e., the outercover, if necessary, 'being modified by cutting away the beads and partof the walls, as indicated at 4, to fit the mould.

The snker 10 having been inserted inside the mould an inflatable air bag5 is placed inside the snker together with an annular plate 6.

`It is now possible, by inating the air bag 5, to exert suflicientpressure through the snker 10 to ensure that the strip 1 is applieduniformly throughout its entire perimeter against the bottom of themould. Because of its rigidity, the snker will ensure that no impressionof 'the mould pattern, i.e., the projection 7, will appear on thereverse side of the tread band 1. On completion of this operation theair bag 5 is deflated and the snker and ring 6 removed.

The procedure for retreading a rigid breaker tire can be sumrned up asfollows:

(1) Place -the outer cover to be retreaded on the spreader and bufi? itto the correct size and contour.

(2) Place the new unvulcanised tread strip into the matrix or mould sothat it rests on the projections forming the tread pattern in the mould.

(3) Place the tread snker over the tread strip.

(4) Insert a pressure bag and a support rim inside the tread snker andinate the bag. (At this point, the mould may also be wanned to assistthe new rubber being completely pressed home into the interstices of themould).

(5) Defiate the bag and remove it together with the ;support rim andsnker.

(6) Insert the outer cover and proceed in the usual way. As the tire hasbeen buffed to the correct size and the unvulcanised tread strip hasbeen completely forced into the tread pattern, only a fractionalmovement is needed to obtain the necessary pressure between the surfaceto be bonded to one another.

What is claimed is:

In a method of retreading a rigid breaker tire having reinforcementmembers which are relatively inextensible Compared with textile threadsused as reinforcement in conventional tires, which rigid breaker tire ishighly resistant to extension and compression along its circumference,the steps of (a) inserting into a circumferential matrix having a treadpattern a strip of tread band material to form t-he new tread, saidmatrix being symmetrical with respect to a central plane, and said stripof tread band material being substantially wider transversely to saidplane than it is thick radially of said matrix and, when inserted intosaid matrix, extending transversely substantially equ-ally to both sidesof said plane;

(b) inserting into the matrix the carcass of a conventional breaker tireof substantially the same size as the rigid breaker tire to beretreaded, said conventional breaker tire carcass having a smoothperipher al outer surface and being relatively extensible along itscircumference as compared to the rigid breaker -tire to be retreaded;

(c) applying pressure to the inside of said carcass to expand the latterinto forceful contact wit-h said strip of tread material and thereby toforce said strip into intimate forceful contact with the inside of saidmatrix, said carcass having sufficient rigidity to ensure that noimpression of the matrix pattern Will be formed on the inside of saidstrip of tread band material When said carcass has been expanded andthen removed from said matrix;

(d) removing said relatively circumferentially extensible carcass fromsaid matrix;

(e) insertng the rigid breaker tire to be retreaded into said matrix andcircumferentially within said strip of tread band material in saidmatrix; and

(f) vulcanizing said strip of tread material to said rigid breaker tire.

References Cited UNITED STATES PATENTS 2,110,293 3/ 1938 Fischer.2,497,226 2/ 1950 McNeill 264-326 XR FOREIGN PATENTS 899,702 6/ 1962Great Britain.

ROBERT F. WHITE, `'Pr'may Exam'ner.

T. J. CARVIS, Assistant Exam'ner.

